Medical stent with a valve and related methods of manufacturing

ABSTRACT

Medical stents having valves and their methods of manufacture are disclosed. The valve may be basket-shaped and formed integral to a medical stent to prevent undesirable backflow across the valve. The valve can be formed by converting the braided wires of the stent, by providing elastomeric material onto a mold or fixture to form an elastomeric valve, or by attaching a gasket valve. The valve is normally closed but configured to allow easy opening in response to a predetermined condition.

DESCRIPTION OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to medical stents and methods ofmanufacturing the same. In particular, the present invention relates tomedical stents with valves for preventing harmful gastric acid reflux ina patient.

[0003] 2. Background of the Invention

[0004] Medical stents are generally flexible, tubular, expandable bodiesformed of a plurality of interconnecting wires. The stents are used in awide variety of medical applications, such as treatment of esophagealdiseases or reinforcing constricted blood vessels or urinary tracts. Thestent is usually placed into a constricted portion of a patient's bodyusing a delivery system, e.g. a catheter.

[0005] When a medical stent is used for treatment of an esophagealdisease, such as esophageal tumor or stricture, the stent is placed atthe lesion within the esophagus to maintain the esophageal lumen open.If the tumor or stricture is located near the junction between thestomach and the esophagus, the esophageal stent is often implantedacross the lower esophageal sphincter (i.e. the ring-like muscle thatconstricts and relaxes the esophagus as required by normal physiologicalfunctions). However, the implantation of a stent across thenormally-closed esophageal sphincter may hold the sphincter openunintentionally and cause harmful gastric acid reflux from the stomachinto the esophagus.

[0006] In order to reduce the gastric acid reflux, it has been proposedto use an anti-reflux valve with an esophageal stent. An example ofesophageal stent with an anti-reflux valve is disclosed by Köcher et al.(“Esophageal Stent with Antireflux Valve for Tumors Involving theCardia: Work in Progress,” JVIR 1998; 9:1007-1010). The anti-refluxvalve of Köcher et al. is made of a pliable, soft polyurethane sleeveattached to the lower end of the stent. However, there are severalproblems associated with this type of stent. For example, the sleevemust be long enough to prevent the reflux and act as a barrier wall todefeat capillary flow of acid up the bore of the device. Since thesleeve must be long, greater deployment force and more complex deliverycatheter designs are required. Typically, the length of the sleeveranges from about 50 to 120 mm and requires extra length on the deliverysystem to envelope it in the “folded” condition prior to deployment. Inaddition, the sleeve may become twisted, tangled, or kinked, which mayinhibit the passage of food into the stomach. The sleeve also may becomereversed and pushed up into the esophagus during vomiting. In that case,it may be difficult for the sleeve to return to its properly functioningposition.

SUMMARY OF THE INVENTION

[0007] To overcome the drawbacks of the prior art and in accordance withthe purposes of the invention, as embodied and broadly described herein,one aspect of the invention provides an esophageal medical stent havinga rigid but elastic valve formed, preferably, near a distal end portionof the stent. The valve is normally closed but configured to open inresponse to a predetermined condition. For an esophageal stent, thepredetermined condition may be a pressure difference between theupstream and the downstream of the valve. The normally closed valve thenallows easy opening of the valve when the pressure difference exceeds apredetermined threshold value. For instance, a passage of food from theesophagus into the stomach causes the pressure difference across thevalve large enough to open the valve and, upon completion of the foodpassage, the valve returns to its normal-closed state to prevent thereflux. A reverse backflow due to, for example, vomiting, which causes alarge pressure difference, may also be permitted by configuring thevalve with an appropriate threshold value for the reverse backflow.

[0008] According to another aspect of the present invention, theesophageal stent of the present invention may also be used for treatmentof the gastroesophageal reflux disease (GERD). GERD is a frequentbackflow of harmful gastric acid from the stomach into the esophagus.When the lower esophageal sphincter inadvertently relaxes atinappropriate times, e.g. after meals, it allows acid and food particlesto reflux back into the esophagus. Although most of the reflux contentsreturn back to the stomach, the remaining gastric acid reflux irritatesthe wall of the esophagus and produces discomfort or pain known asheartburn. GERD, however, is a medical condition when such reflux isfrequent or severe enough to cause more significant problems. In orderto treat GERD, a stent having an anti-reflux valve of the presentinvention can be placed in the lower esophagus to prevent the harmfulgastric acid reflux.

[0009] Another aspect of the present invention, therefore, is to providea method of manufacturing a medical stent having an valve. The methodincludes: providing a generally tubular body formed of braided wires andhaving a proximal end portion and a distal end portion; extending thebraided wires near the distal end portion; and deforming the extendedwires to form the valve, wherein the valve is configured to be normallyclosed and to be open in response to a predetermined condition. Thevalve is formed basket-shaped and at least a portion of the valve and/orat least a portion of the tubular body are provided with a suitablecovering material.

[0010] Another aspect of the present invention is to provide a method ofmanufacturing a medical stent having an elastomeric valve. The methodincludes: providing a generally tubular body; positioning a fixtureproximate to a portion of the tubular body; applying an elastomericmaterial onto the fixture; and removing the fixture to form theelastomeric valve, wherein the elastomeric valve is configured to benormally closed and to be open in response to a predetermined condition.

[0011] In yet another aspect of the present invention, a method ofmanufacturing a medical stent having a gasket valve includes: providinga generally tubular body; and attaching an elastomeric gasket valveintegral to a portion of the tubular body, wherein the integral gasketvalve is configured to be normally closed and to be open in response toa predetermined condition.

[0012] In still another aspect of the present invention, a medical stentcomprises: a generally tubular body formed of braided wires and having aproximal end portion and a distal end portion; and a normally closedvalve formed from the braided wires extended from the distal endportion, wherein the valve is configured to open in response to apredetermined condition. The valve is a basket-shaped spring valve, andat least a portion of the tubular body and/or the valve is provided witha suitable covering material.

[0013] In still another aspect of the present invention, a medical stentcomprises: a generally tubular body having a proximal end portion and adistal end portion; and a normally closed valve made of an elastomericmaterial and formed integral to the distal portion of the tubular body,wherein the elastomeric valve is configured to open in response to apredetermined condition. The elastomeric valve is basket-shaped, and atleast a portion of the tubular body and/or the valve is provided with asuitable covering material.

[0014] Additional objects and advantages of the invention will be setforth in part in the description which follows, and in part will beobvious from the description, or may be learned by practice of theinvention. The objects and advantages of the invention will be realizedand attained by means of the elements and combinations particularlypointed out in the appended claims.

[0015] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate the variousembodiments of the invention and, together with the description, serveto explain its advantages and principles.

[0017] In the drawings:

[0018]FIGS. 1A and 1B are perspective views of embodiments of medicalstents with valves, according to the present invention;

[0019]FIG. 2 is a schematic diagram showing a method of manufacturingthe embodiment shown in FIG. 1A, according to an embodiment of thepresent invention;

[0020] FIGS. 3A-D are side and cross-sectional views of variousembodiments of medical stents with valves, according to the presentinvention, showing the open and closed states of the valves;

[0021]FIG. 4 is a schematic diagram showing a method of manufacturingthe embodiment shown in FIG. 1B, according to an embodiment of thepresent invention;

[0022]FIG. 5 is side and cross-sectional views of embodiments of medicalstents with valves, according to the present invention, showing the openand closed states of the valves; and

[0023]FIG. 6 is side and cross-sectional views of another embodiment ofa medical stent with a valve, according to the present invention,showing the open and closed states of the valve.

DESCRIPTION OF THE EMBODIMENTS

[0024] Reference will now be made in detail to the present embodimentsof the invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

[0025] Referring to FIG. 1A, a stent 1 according to an embodiment of thepresent invention includes a self-expanding tubular body 2 having aproximal end portion 4 and a distal end portion 5. The tubular body 2 isformed by braiding or knitting a plurality of flexible wires 3 orfilaments to provide sufficient radial expansion force. The wires 3 orfilaments can be made of metal, polymeric materials, composites thereof,or other suitable materials known in the art which exhibit sufficientelasticity, such as a memory material like nickel titanium alloy (i.e.nitinol). The tubular bodies 2 in the exemplary figures are of similarshape, i.e. funnel-like shape having the cross-sectional area in theproximal end portion being greater than the one in the distal endportion. The shape of the tubular bodies 2 in the exemplary figures are,however, not meant to limit or narrow the scope of the presentinvention. One of ordinary skill in the art would recognize that othertypes and shapes of tubular bodies known in the art also may be used inthe practice of the present invention.

[0026] At least a portion of the tubular body 2 is provided with astrong covering 6 made of an elastic material such as polyurethane,silicone, polytetrafluoroethylene (i.e. teflon), or other suitablematerial exhibiting sufficient strength characteristics. The covering 6functions primarily as a barrier to resist tumor or other tissueingrowth.

[0027] Near the distal end portion of the tubular body 2, abasket-shaped valve 10 is formed integral to the tubular body 2. Thevalve 10 is normally closed to prevent acid reflux, but configured toopen for the passage of food from the esophagus into the stomach. Theformation of the valve 10 can be carried out before or after heattreatment, or other final manufacturing steps, of the stent 1.

[0028] A method of forming the valve 10 according to an embodiment ofthe present invention is illustrated in FIG. 2. In this method, thebraided wires 3 are extended from the distal end portion 5 of thetubular body 2 to form distal wires 8. The extended distal wires 8 arethen deformed into a basket-shaped valve 10. Deforming the distal wires8 is performed, for example, by curling the distal wires 8 inwards asshown in FIG. 2. Each of the distal wires 8 is wound around and thenreleased from a cylindrical member 9 so that the distal wires 8 form abasket-shaped spring valve 10, by curling toward the center 12 of thecircle 14 formed by the tubular body 2 (See FIG. 3A). FIG. 3A shows theside and distal end views of the embodiment of FIG. 1A. The distal endviews include the open state (left-hand side) and closed state(right-hand side) of the valve. The lower end view figures of FIG. 13Ashow that the curled wires may also be curled sideways.

[0029] According to another embodiment of the present invention, shownin FIGS. 3B and 3C, the valves 10 a, 10 b are formed by straighteningthe distal wires 8 and bending each of the distal wires 8 inwards at apredetermined location 11, 11′ of the distal wires 8.

[0030] According to still another embodiment of the present invention,shown in FIG. 3D, the valve 10 c is formed by curling each of the distalwires 8 outward with the middle portion 13 of each of the distal wires 8converged toward the center 12 of the circle 14 formed by the tubularbody 2 such that each of the bent distal wires 8 forms a U-shaped wire.

[0031] The valves 10, 10 a, 10 b, 10 c also may be covered on at least aportion of the valve 10, 10 a, 10 b, 10 c with an elastic coveringmaterial to function as a barrier to the reflux. The covering materialis selected from a group of polyurethane, silicone, andpolytetrafluoroethylene (i.e. teflon), or other suitable materialsexhibiting similar characteristics. Preferably, the same material usedto cover the tubular portion 2 is used for the valve covering material.The covering 6′ of the valves 10, 10 a, 10 b, 10 c may be loose orpleated in order to not inhibit opening and closing of the valve 10, 10a, 10 b, 10 c.

[0032] The right-hand side of FIGS. 3A-3D show cross-sectional views ofthe valves 10, 10 a, 10 b, 10 c looking down from the cross-sectionalplane A-A′, which illustrate the open and closed states of the valves10, 10 a, 10 b, 10 c. The valves 10, 10 a, 10 b, 10 c are normallyclosed but are configured to open in response to a predeterminedcondition. The predetermined condition may be a pressure differencebetween the upstream and the downstream of the valves 10, 10 a, 10 b, 10c, so that the valves can open while the pressure difference exceeds acertain threshold value, e.g. one induced by a passage of food throughinto the stomach. The adjustment of the threshold value may beperformed, for example, by adjusting the number of distal wires 8 usedto form the valves 10, 10 a, 10 b, 10 c. The number of distal wires 8can be tailored to create a certain amount of spring force that allowsthe food to pass though the stent 1 into the stomach and also resiststhe backward reflux pressure. The threshold value also may be adjustedby changing the material of the stent and wires, or the covering. Whilethe valves 10, 10 a, 10 b, 10 c can be configured to be one-way valves,it may be beneficial to allow occasional reverse opening caused by, forexample, vomiting, by configuring the valve (e.g. adjusting the springforce required to open the valve) with an appropriate threshold valuefor a reverse opening.

[0033] The length of the valve ranges from 5 to 50 mm, preferably from10 to 30 mm. The length of the valve is relatively short compared toother types of anti-reflux valves. Therefore, the valve of the presentinvention requires relatively lower deployment force and, thereby, usesless complex delivery systems.

[0034]FIG. 1B shows a stent 1′ having an elastomeric valve 20 accordingto still another embodiment of the present invention. In thisembodiment, a basket-shaped elastomeric valve 20 made of an elastomericmaterial is formed integral to the distal end portion of the tubularbody 2. Although the tubular body 2 is formed of knitted or braidedwires in the exemplary figure, one skilled in the art would recognizethat other types of tubular bodies known in the art may also be utilizedin the present invention.

[0035] The elastomeric valve 20 functions similarly to the valveillustrated in FIG. 1B. The valve 20 is normally closed to prevent acidreflux but openable for the passage of food from the esophagus into thestomach. At least a portion of the tubular body 2 is provided withstrong covering 6 made of an elastic material such as polyurethane,silicone, or polytetrafluoroethylene (i.e. teflon). The valve 20 may beprovided with a small opening 21 at the bottom of the valve 20, and theopening 21 may have pleats or cuts 22 to facilitate easy opening of thevalve 20. The purpose of the opening 21 is to allow the flow of gas fromthe stomach up through the esophagus so as to allow a patient to burpand relieve gas pressure in the stomach. However, the opening 21 issmall enough to prevent the passage of significant amount of liquid orsolid. The diameter of the opening 21 generally ranges from 1 to 10 mm,preferably around 3 mm.

[0036] A method of forming the elastomeric valve 20 is schematicallyillustrated in FIG. 4. A mold or fixture 15 for a shape of theelastomeric valve 20, preferably basket-shaped, is attached to thedistal end portion 5 of the tubular body 2. The fixture 15 is preferablyinserted from the proximal end portion 4 with an elongated handle 16. Onthe surface of the fixture 15, a protrusion 19 is formed to provide asmall opening 21 in the elastomeric basket valve 20. Once the fixture 15is fixed to the tubular body 2, the fixture 15 is coated with anelastomeric material 18, preferably the same material used for coveringthe tubular body 2 of the stent. The coating can be performed, forexample, by spraying 17 or dip-covering (not shown) the elastomericmaterial 18 onto the surface of the fixture 15. Preferably, this step isperformed simultaneously with a step of covering the tubular portion 2with the covering material 6. After the coating, the elastomericmaterial 18 coated on the surface of the fixture 15 is cured and thefixture 15 is removed from the stent 1′. The elastomeric valve 20 havingthe shape of the fixture 15 is thereby formed. Although the exemplaryembodiment illustrates the valve 20 attached at the distal end portion,one skilled in the art would recognize that the valve can be formedanywhere in the tubular body 2 between the proximal end portion 4 andthe distal end portion 5.

[0037] While the elastomeric valve 20 does not have the structuralreinforcement of the wires 3, the valve 20 is relatively rigid and, atthe same time, sufficiently elastic to allow it to stretch open whilefood passes through the stent 1′ into the stomach and to spring back andclose to prevent the reflux. The valve 20 may be provided with pleats orslits 22 to facilitate opening of the valve 20. FIG. 5 is the side andcross-sectional views of embodiments of medical stents with valves. Theupper and lower figures on the right-hand side of in FIG. 5 show theopen and closed states of the valve 20 having the opening 21 with andwithout slits 22, respectively. While the slits 22 facilitate theopening of the valve 20, the valve 20 either with or without the slits22 on the opening 21 is capable of functioning in the manner describedabove.

[0038] The rigidity and elasticity of the elastomeric valve 20 can betailored by carefully selecting characteristic parameters, such as thetype of elastomeric material 18, coating thickness, number of slits 22,size of hole 21, treatment processes, etc.

[0039]FIG. 6 shows a stent 1″ with an integral gasket valve 30 accordingto still another embodiment of the present invention. The term “gasketvalve” is defined as a flat piece of “gasket” material (e.g., a polymeror elastomer) made to function as a valve by providing pleats or cuts init. In this embodiment, a basket-shaped gasket valve 30 made of anelastomeric material is integrally adhered, sewed, or injection-moldedto the distal end portion 5 of the tubular body 2. The integral gasketvalve 30 is made of, preferably, the same material 6 used for thetubular portion 2, such as polyurethane, silicone,polytetrafluoroethylene (i.e. teflon), or other suitable materialexhibiting similar characteristics.

[0040] A flexible gasket valve used in a typical vascular introducersheath for interventional radiology may be used as the integral gasketvalve 30. The integral gasket valve 30 also functions similarly as thebasket valves illustrated in FIGS. 1A and 1B, i.e. normally closed toprevent acid reflux but opened for passage of food from the esophagusinto the stomach. A small opening 21 is provided at the center of thegasket valve 30 and may be provided with pleats or cuts 22 to facilitateeasy opening of the valve 30.

[0041] Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A method of manufacturing a medical stent havinga valve, comprising: providing a generally tubular body formed ofbraided wires and having a proximal end portion and a distal endportion; extending the braided wires near the distal end portion; anddeforming the extended wires to form the valve, wherein the valve isconfigured to be normally closed and to be open in response to apredetermined condition.
 2. The method of manufacturing the medicalstent according to claim 1, wherein the valve is basket-shaped.
 3. Themethod of manufacturing the medical stent according to claim 1, furthercomprising a step of providing a covering to at least a portion of thetubular body.
 4. The method of manufacturing the medical stent accordingto claim 3, wherein the material for the covering is selected from agroup of polyurethane, polytetrafluoroethylene, and silicone.
 5. Themethod of manufacturing the medical stent according to claim 1, furthercomprising a step of providing a covering to at least a portion of thevalve.
 6. The method of manufacturing the medical stent according toclaim 5, wherein the material for the covering is selected from a groupof polyurethane, polytetrafluoroethylene, and silicone.
 7. The method ofmanufacturing the medical stent according to claim 1, wherein deformingthe extended wires is performed by curling the extended wires inwards.8. The method of manufacturing the medical stent according to claim 1,wherein deforming the extended wires is performed by straightening theextended wires and bending the extended wires inwards at a predeterminedlocation of the extended wires.
 9. The method of manufacturing themedical stent according to claim 1, wherein deforming the extended wiresis performed by curling the extended wires so that middle portions ofthe extended wires converge toward each other.
 10. The method ofmanufacturing the medical stent according to claim 1, wherein thepredetermined condition is a predetermined pressure difference betweenan upstream and a downstream of the valve.
 11. The method ofmanufacturing the medical stent according to claim 1, wherein the valveis configured to function as a one-way valve.
 12. A method ofmanufacturing a medical stent having an elastomeric valve, comprising:providing a generally tubular body; positioning a fixture proximate to aportion of the tubular body; applying an elastomeric material onto thefixture; and removing the fixture to form the elastomeric valve, whereinthe elastomeric valve is configured to be normally closed and to be openin response to a predetermined condition.
 13. The method ofmanufacturing the medical stent according to claim 12, wherein theelastomeric valve is basket-shaped.
 14. The method of manufacturing themedical stent according to claim 12, further comprising a step of curingthe applied elastomeric material.
 15. The method of manufacturing themedical stent according to claim 12, further comprising a step ofproviding a covering to at least a portion of the tubular body.
 16. Themethod of manufacturing the medical stent according to claim 15, whereinthe step of providing the covering is performed simultaneously with thestep of applying the elastomeric material.
 17. The method ofmanufacturing the medical stent according to claim 15, wherein amaterial for the covering and the elastomeric material are the same. 18.The method of manufacturing the medical stent according to claim 12,wherein the elastomeric material is selected from a group ofpolyurethane, polytetrafluoroethylene, and silicone.
 19. The method ofmanufacturing the medical stent according to claim 12, wherein thepredetermined condition is a predetermined pressure difference betweenan upstream and a downstream of the elastomeric valve.
 20. The method ofmanufacturing the medical stent according to claim 12, wherein theelastomeric valve is configured to function as a one-way valve.
 21. Themethod of manufacturing the medical stent according to claim 12, whereinthe fixture is attached to a distal end portion of the tubular body. 22.The method of manufacturing the medical stent according to claim 12,wherein the elastomeric valve is formed so that the valve has an openingin the normally closed state.
 23. The method of manufacturing themedical stent according to claim 22, the opening includes at least oneslit.
 24. A method of manufacturing a medical stent having a valve,comprising: providing a generally tubular body; and attaching anelastomeric gasket valve integral to a portion of the tubular body;wherein the gasket valve is configured to be normally closed and to beopen in response to a predetermined condition.
 25. The method ofmanufacturing the medical stent according to claim 24, wherein the valveis attached to a distal end portion of the tubular body.
 26. The methodof manufacturing the medical stent according to claim 24, wherein theelastomeric gasket valve is attached to the portion of the tubular bodyby adhesives.
 27. The method of manufacturing the medical stentaccording to claim 24, wherein the elastomeric gasket valve is sewed tothe portion of the tubular body.
 28. The method of manufacturing themedical stent according to claim 24, wherein the elastomeric gasketvalve is injection-molded to the portion of the tubular body.
 29. Themethod of manufacturing the medical stent according to claim 24, whereinthe predetermined condition is a predetermined pressure differencebetween an upstream and a downstream of the valve.
 30. The method ofmanufacturing the medical stent according to claim 24, wherein the valveis configured to function as a one-way valve.
 31. A medical stent havinga valve, comprising: a generally tubular body formed of braided wiresand having a proximal end portion and a distal end portion; and anormally closed valve formed from the braided wires extended from thedistal end portion, wherein the valve is configured to open in responseto a predetermined condition.
 32. The medical stent according to claim31, wherein the valve is a basket-shaped spring valve.
 33. The medicalstent according to claim 31, wherein at least a portion of the tubularbody is provided with a covering.
 34. The medical stent according toclaim 33, wherein the material for the covering is selected from a groupof polyurethane, polytetrafluoroethylene, and silicone.
 35. The medicalstent according to claim 31, wherein at least a portion of the valve isprovided with a covering.
 36. The medical stent according to claim 35,wherein the material for the covering is selected from a group ofpolyurethane, polytetrafluoroethylene, and silicone.
 37. The medicalstent according to claim 31, wherein the predetermined condition is apredetermined pressure difference between an upstream and a downstreamof the valve.
 38. The medical stent according to claim 31, wherein thevalve is a one-way valve.
 39. The medical stent according to claim 31,wherein the extended wires forming the valve are curled inwards.
 40. Themedical stent according to claim 31, wherein the extended wires formingthe valve are straightened and bent inwards at a predetermined locationof the extended wires.
 41. The medical stent according to claim 31,wherein the extended wires forming the valve are curled so that middleportions of the extended wires converge toward each other.
 42. A medicalstent having an elastomeric valve, comprising: a generally tubular bodyhaving a proximal end portion and a distal end portion; and a normallyclosed valve made of an elastomeric material and formed integral to thedistal end portion of the tubular body, wherein the elastomeric valve isconfigured to open in response to a predetermined condition.
 43. Themedical stent according to claim 42, wherein the elastomeric valve isbasket-shaped.
 44. The medical stent according to claim 42, wherein theelastomeric valve is a gasket valve.
 45. The medical stent according toclaim 42, wherein at least a portion of the tubular body is providedwith a covering.
 46. The medical stent according to claim 45, whereinthe material for the covering is selected from a group of polyurethane,polytetrafluoroethylene, and silicone.
 47. The medical stent accordingto claim 42, wherein at least a portion of the elastomeric valve isprovided with a covering.
 48. The medical stent according to claim 47,wherein the material for the covering is selected from a group ofpolyurethane, polytetrafluoroethylene, and silicone.
 49. The medicalstent according to claim 48, wherein at least a portion of the tubularbody is provided with a covering and a material for the covering in thetubular body is the same as the material for the covering in theelastomeric valve.
 50. The medical stent according to claim 42, whereinthe predetermined condition is a predetermined pressure differencebetween an upstream and a downstream of the elastomeric valve.
 51. Themedical stent according to claim 42, wherein the elastomeric valve is aone-way valve.
 52. The medical stent according to claim 42, wherein theelastomeric valve includes an opening when the valve is in the normallyclosed position.
 53. The medical stent according to claim 52, whereinthe opening includes at least one slit.